How to reduce the amount of noise in the rendered image?
Some tricks I use:

Avoid lights that have to bounce a lot to light your scene. Instead of a sunny exterior, consider making the window glass a light (ie make the emitter the last bounce in your system).

Use ambient occlusion option in world settings. It will brighten things up but start out less noisy.

How is your scene lit? It looks like there is not enough light. Try taking a photograph with no light it also looks grainy ;)